The invention relates to an X-ray tube having a shaft supporting a rotary anode. The shaft is rotatably journalled by means of a magnetic bearing comprising a rotor connected to the rotary anode and a stator having one or more magnet yokes. Each magnetic yoke intersects the envelope of the tube and is magnetizable by a magnet outside the tube. Each magnetic circuit of the bearing includes gaps between opposed, axially-spaced annular faces disposed on the rotor and on the yoke, respectively, so as to extend transverse to of the shaft.
Such an X-ray tube is particularly suitable for use as the radiation source in medical X-ray diagnosis apparatus.
German Patent Application No. 2,262,757, which corresponds to U.S. Pat. No. 3,878,395, discloses an X-ray tube as described above in which the magnet yoke of the stator comprises a plurality of flat stator rings which extend transverse to the shaft. The rotor comprises a plurality of flat rotor rings which also extend transvese to the shaft. The inside diameter of the stator rings is smaller than the outside diameter of the rotor rings. The rings are arranged such that facing portions of neighboring stator and rotor rings have cooperating faces which are separated by axial gaps. Magnetic flux of magnets arranged outside the tube can penetrate into the interior through these gaps, axial attractive forces are exercised on the rotor, however, these forces are oppositely directed in neighboring gaps. Due to these forces, the rotor is an axial equilibrium which is, however, unstable. The dimensions of the gaps, and consequently the position of the rotor, are kept constant in the axial direction of the shaft by stabilizing this unstable equilibrium. This is achieved by means of an axial journal or ball bearing, so that the rotary anode is axially supported.
Furthermore, the rotor rings have their inner edges arranged axially opposite outer edge of flat rings. These flat rings constitute magnet yokes of magnets which are secured inside the tube and which have been magnetized so that repelling forces are exercised on the rotor rings which are magnetized by the magnets disposed outside the tube. As a result of these repelling forces, the rotor is in stable radial equilibrium, so that the rotary anode is radially supported as well as axially supported.
A great number of components is necessary to obtain adequate support, particularly radial support, of the rotary anode within this known X-ray tube. The specification describes the use of eight stator rings with seven field coils which are disposed outside the tube between the stator rings, eight rotor rings, and eight rings arranged inside the tube and having seven field coils disposed therebetween. These components must be accurately positioned relative to one another within the X-ray tube, which results in a relatively expensive construction. This X-ray tube has the further drawback that the magnetic bearing forces are partly produced by field coils which are fixed inside the tube. During operation of the X-ray tube, these field coils attain a temperature of between 200.degree. C. and 500.degree. C., and will therefore release gas, particularly from the insulating means between the wires, and consequently adversely affect the tube vacuum.